\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}

% To convert the document to a png:
% magick convert -density 200  -trim filename.pdf -quality 100 filename.png


\begin{document}
\begin{tikzpicture}[
latentnode/.style={circle, draw=red!80, minimum size=6mm, very thick},
factornode/.style={draw=red!80, fill=red!80, minimum size=6mm, very thick},
]

\begin{scope}

\node[latentnode, label=above: $x_1$] (x1) {};
\node[factornode, label=below: $f_a$] (fa) [below right=of x1] {};

\node[latentnode, label=above: $x_2$] (x2) [above right=of fa] {};
\node[factornode, label=below: $f_b$] (fb) [below right=of x2] {};

\node[latentnode, label=above: $x_3$] (x3) [above right=of fb] {};
\node[factornode, label=below: $f_c$] (fc) [below right=of x3] {};
\end{scope}

\end{tikzpicture}

\end{document}
